CSP regulation of exo-endocytic cycle enhances synaptic stability. Synapses are intricate structures that undergo structural modifications constantly. In healthy brains, synapses are maintained by activity-dependent mechanisms. These processes are compromised in neurodegenerative diseases such as Alzheimer's and Parkinson's diseases, leading to profound synapse loss early in disease progression. The purpose of this project is to investigate presynaptic mechanisms of synapse maintenance using a mouse lacking the co-chaperone cysteine string protein (CSP). The nervous system of this mouse develops normally, however synapses are rapidly lost after maturation of the mouse, leading to gross neurodegeneration and early death. We have performed an unbiased screen for CSP clients, which indicates that CSP is interacting with select proteins involved in synaptic vesicle exo- and endocytosis. We therefore hypothesize that CSP stabilizes synapses by regulating the exo-endocytic cycling of synaptic vesicles and interacting with the presynaptic cytoskeleton. I will test our hypothesis by first examining activity-dependent synaptic vesicle cycling using stimulated neuron cultures, endocytic labeling and electron microscopic analysis. Next, I will establish an expanded list of CSP client proteins using pulldown experiments followed by proteomic analysis. Finally, using the list of client proteins, I will test whether any single client or a combination of clients are able to modify the synapse loss phenotype observed in CSP knockout neurons using overexpression and knockdown techniques. This study will provide insight into how synapses are maintained in healthy nervous systems and how synapses are lost in neurodegenerative diseases.